a) Field of the Invention
The present invention relates generally to an electronic control for a vehicle four wheel drive system and, in particular, to an electronic control which includes means for detecting a predetermined slip (speed differential) between the front and rear wheels of the vehicle, and means for selectively engaging a clutch during excessive slip conditions to prevent relative slip between the front and rear wheels.
b) Description of Related Art
Four wheel drive systems for vehicles are becoming increasingly common. In the past, such systems typically included torque transfer case having an input shaft connected to the output of the vehicle driver for selecting front and rear output shafts connected to the front and rear differentials of the vehicle for driving the front and rear wheels respectively. Typically, such systems were provided with selective control means operable by the vehicle driver for selecting whether the vehicle is to be operated in either a two wheel or a four wheel drive mode. When operated in the four wheel drive mode, these systems did not provide for any speed differentiation between the front and rear wheels such that, on dry pavement, xe2x80x9choppingxe2x80x9d of the front wheels would occur during turning of the vehicle, due to the normal overspinning of the front wheels. Consequently, it was recommended that the four-wheel mode be used only during wet, icy, or low traction road surface conditions.
Recently, certain vehicles have been provided with a xe2x80x9cfull timexe2x80x9d four wheel drive system. In these systems, the torque transfer cases are typically provided with an interaxle differential for dividing torque between the vehicle front and rear differentials. The interaxle differential enables the front and rear wheels to rotate at different speeds, which occurs during turning of the vehicle, or in the event the front and rear wheels have different diameter tires. Also, in order to prevent excessive slipping between the front and rear wheels, these transfer cases typically include a selectively engageable clutch means which is operative to lock the interaxle differential upon sensing a predetermined slippage between the front and rear output shafts of the transfer case.
However, the automatic control systems for these selectively engageable clutches has many drawbacks that increase cost without provide accurate and efficient activation of the selectively engageable clutches.
Current torque coupling clutch control systems require separate algorithms to perform low-speed and high speed detection of wheel slip. Moreover, current control systems provide only periodic sampling of rotational speeds differences in the torque coupling transmission.
Current control systems also do not permit modularization of the control system, allowing processing and decision making to be broken up onto separate, smaller processors that would be required if all of the processing occurred at one central processor.
The need therefore exists for a torque coupling control system that overcomes the drawbacks inherent in the prior art, while providing a controller that performs low and high speed detection of wheel slip with a single algorithm that continuously assesses operation of the drivetrain.
The present invention concerns a microcomputer-based electronic control system for automatically controlling a full-time four wheel drive torque transfer case or similar drivetrain device. The transfer case includes an input shaft coupled to the output of the vehicle transmission, and an interaxle planetary gear differential for dividing torque between a rear output shaft connected to the vehicle rear differential and a front output shaft connected to the vehicle front differential. An electromagnetic clutch is located in the transfer case and is adapted to selectively lock the planetary gear differential to prevent relative slip between the front and rear output shafts.
In accordance with the present invention, the electronic control utilizes a pair of Hall effect sensors for monitoring the speed of the front and rear output shafts. When a predetermined amount of slip is detected between the front and rear output shafts, the electronic control is operative to selectively engage the electromagnetic clutch for a predetermined time period. During this predetermined time period, the clutch is engaged to lock the differential and prevent slip between the front and rear output shafts.
The electronic control of the present invention includes several unique operating features. For example, the system of this invention is capable of detecting the relative rotational displacement of one shaft with respect to another at all speeds down to zero revolutions per minute.
The control system continuously interrogates two Hall effect sensors positioned over toothed wheels on the front and rear outputs of the transfer case. Tooth counts are stored in two numerical stacks (one for each sensor), the depth of which may be adapted to the requirements of the particular design. Upon detecting a tooth count, the system adds the count to the sensors current stack register and pushes all of the registers of the other stack down one register. The last register is pushed out or deleted from the stack memory. In this way, the system retains a continuous memory of tooth counts from one sensor for the last xe2x80x98nxe2x80x99 tooth counts of the other, and vice-versa. By comparing the sum of the tooth counts from each stack, the relative rotation of one shaft to the other shaft is directly ascertained independent of any time-based reference. Thus, the system detection relative rotational differences at all speeds down to zero rpms. In addition, because of an equal probability of missing tooth counts, the system works well above the maximum sampling rate of the system.
The system of this invention further permits modularization of the controller, thus allowing the processing and decision-making function to be broken up onto separate, smaller processors than would be required if all of the processing occurred on one central processor. This modularization may decrease the overall cost of the control system.
The above features, as well as other advantages of the present invention, will become readily apparent to one skilled in the art from reading the following detailed description in conjunction with the attached drawings.